Evolution of Design
I started the electrical design with some preconceived notions about how it was going to work, but that’s not at all where I ended up. I could take you through the whole journey, but unless you are really into amps and amp hours the trip would probably put you to sleep. Let me try to condense it down and spend most of my time (your time), on where I ended up and why I went there.
My initial thought was that an air conditioner draws too much current to be powered from an inverter, or more to the point, from batteries. Therefore I resolved to plug the air conditioner directly into the generator, and then use the left over current from the generator to charge the batteries.
Another assumption was that I would end up using a more or less standard RV style Inverter – Charger – Transfer switch.
I also started by using a design philosophy that I got from “Managing 12V” by Harold Barre, that basically involves calculating your energy requirements for an entire day (or whatever time period you choose) and then providing enough battery capacity to meet your needs. No offence to Harold Barre, whose ideas are perfectly valid. But none of these things worked for me.
Lets start with plugging the air conditioner directly into the generator. I had been really struggling with the issue of how to deal with the highly variable load of the air conditioner. It’s well within the range of my generator when it is running, but the startup current whenever the compressor turns on can be really high, and it also varies with outside temperature. That and the fact that I just can’t accurately predict it based on the manufacturers data.
I really hate that there is all this leftover generator capacity that I can’t use because I never know when the air conditioner is going to need all of the available capacity, and possibly even more. The highly variable load could easily damage my generator over time, possibly shortening its life. It could also force me to buy a 2nd generator, for another $1,000 or so, and then I would have even more capacity that I can’t entirely count on.
Finally it hits me, there is another option. I know I said I never want to run the air conditioner off of the batteries, and I still don’t, but I might just want to run it off of the inverter. It goes something like this. I buy a 90A battery charger, and then connect the battery charger, and only the battery charger, to the output of the generator. The generator is rated at 13.3A continuous, so it should be able to drive the battery charger (13A) all day long.
The output of the battery charger is then connected to the input of a high power inverter which in turn drives the air conditioner and all of the various AC appliances. It also connects to the batteries of course.
At full running load in desert conditions (120F) the Air conditioner needs 1,260W. The maximum load for all the other AC appliances turned on all at once, including the microwave is 1,387W. Altogether that’s a total of 2,647W. As a practical matter it would probably never go above 2,500W. If I just promise to never run the microwave when the air conditioner is on it drops to 1,847W.
There are many battery chargers in the 90A range from $400 to $600. Some are more true multi-stage chargers and others are more like power supplies. Some are more ruggedized for mobile use and some tend to be more for fixed installations.
There are also many inverters available in this range from a few hundred dollars to around $1,000 depending on features. It might even be handy to use two smaller 1,500W inverters, that way the one that provides power to the air conditioner can be disconnected from the batteries when not in use, so it doesn’t constantly drain the batteries even when providing no power.
There are also battery chargers, inverters, and transfer switches all in one nice neat robust package, but I will get into why that doesn’t work later.
At 2,500W the current draw from the batteries would be about 241A. That’s a heck of a lot of current and would draw a 440Ah (Amp Hours) battery pack flat in under 2 hours. Since I would never want to draw the batteries below about 50% the holdup time is actually less than an hour.
As a practical matter I would not want to even pull that kind of a load from a 440Ah battery pack since it would exceed 25% of the Amp Hour rating (110A). Thanks to something called the Peukert Effect a lead acid battery being discharged in 4 hours or less has much less energy capacity than the nominal 20 hour rating. Put another way, if you draw too much current from the battery then a lot of energy gets wasted heating up the battery.
As a practical matter, I would never turn on the air conditioner, microwave, and everything else all at the same time. A more likely maximum current, and even this is on the high side is 15.5A AC which translates to about 158A DC. That’s still a lot, but it’s getting more reasonable.
If I start the generator first, before the air conditioner, the battery charger is ready and waiting to put out up to 90A. It might not put out the full current at first, but it would very quickly sense that the batteries are drawing down and begin to provide full current. 158A load, minus 90A from the charger leaves 68A to come from the battery. That is well within the 25% rule or 110A. At that rate the batteries would only last about 6 hours, but since I can only use 1/3 of their capacity it’s more like 2 hours. That sounds pretty bad, but this is a worst case instantaneous current calculation.
If I turn off all the computers, monitors, PlayStation3, etc. and just hang out and read a book in air conditioned comfort, I wont need any AC current except for the air conditioner. That’s about 1,200W on low cool, even at 120F outside. But wait, If I have done a half decent job with my insulation then the air conditioner compressor will only be on about half the time, or hopefully less. It takes about 137W to run the fan on low cool, so on average my air conditioner will be drawing 668W, or 6.1A AC, or 56A DC.
There’s probably another 5.5A of DC for the refrigerator and what not, for a total of less than 61A DC. That means my battery charger is actually putting 90A minus 61A, or about 29A back into my batteries. My son can fire up the PS3 (Which is a power hog) and we are pretty much breaking even. Now if the generator quits, I better turn off the air conditioner or get the generator started up again within an hour or so, but it’s not a panic.
The point of all this is that my generator is churning along at something like 90% of its capacity (depending on the choice of battery charger) and that load doesn’t vary when the air conditioner compressor comes on, or pretty much ever. It might reduce as the batteries approach full charge, but it never exceeds the maximum dictated by the charger. If the load on the generator does change, it will change slowly because the batteries can’t be charged or discharged very quickly. That means I can leave the generator in economy mode and it will happily do whatever it needs to do to feed the charger.
The downside is cost. The battery charger is close to what I would have wanted anyway, but the inverter has to roughly double in capacity. That’s another $200 to $500 depending on the specific choices. There are a few other odds and ends, like heavier DC cables, but it’s probably in the noise. Within reason, I will spend money to reduce risk, and I like this tradeoff.
Another downside is efficiency. Converting AC-DC-AC drops my efficiency to about 85% (.90 x .95). If the energy goes into the battery and then comes back out again it’s more like 73% (12 / 14 x .90 x .95). Throwing away over ¼ of the precious energy from my generator sucks, but that’s life.
When shore power is available I just connect it to the battery charger and directly to the air conditioner. That way the batteries are sure to be fully charged at all times. One side benefit of converting from AC-DC-AC is that I have lots of isolation from power surges or what not.
I would still probably buy a 2nd generator though. I wouldn’t need it. It would strictly be a back up. There I go spending money to reduce risk again. Now I can start worrying about the reliability of all these components. There is always something to worry about.
Some battery chargers I found on line that might fill the bill include:
Iota Smart Battery Charger / Converter, 90 amps, 12V, Model #:CHG-DLS-90, Maximum AC Current @108Vac: 13 amps, $369.
http://www.theinverterstore.co...hp?model=chg-dls-90#
In order to make it a 3 stage battery charger and not just a DC Power supply you also need the Iota Smart Controller for DLS Battery Chargers, Model #:CHG-IQ-4, $29.00. The price is very tempting, but I worry that it lacks temperature compensation, and the 13.6V maximum output voltage would likely charge the batteries very slowly as they near full charge. Still, it looks like a worthy option.
If I want a little more safety margin I can use the 80 AMP 12V Battery Charger, by Samlex SEC-1280A, $609.
http://www.donrowe.com/battery...arger_12v_2bank.html
The Samlex appears to be more of a true 3 stage battery charger with temperature compensation and a higher boost voltage (14.4V), which is probably why it is more expensive. The maximum AC input current isn’t specified, but based on the lower output current it is probably about 11A.
To be continued…